In the production of edible oils from oil-bearing seeds, e.g. soya beans, cocoa beans, sesame seed and the like, it has already been proposed to provide processes in which the oil pressing is carried out in a so-called filter press of the worm type. In such systems it has also been suggested to carry out the pressing in a plurality of stages with a pressure reduction following each stage.
A worm or screw press for carrying out such a process is described, for example, in German open application - Offenlegungsschrift - DT-OS 1,944,642.
In this system, an adjustable control or limitation of the dynamic pressure is provided for a progressive pressure increase within the press.
This pressure increase is a result of a progressively reduced thread or helix cross section for each individual stage such that the press pressure ahead of throttle between the stages, gradually increases and immediately upstream of the throttle is reduced to a lower value corresponding to the value at which the pressure rise of the next stage is to commence.
As a result of this pressure development throughout the press, all of the particles of the material remain essentially in the same position vis-i-vis other particles practically throughout their travel through the press.
In practice, it has been found that after the material has been subjected to the first pressure peak in the initial stage, the application of higher pressures to obtain higher pressure peaks in subsequent stages gives rise to only a minimum improvement in the pressing effectiveness. As a practical matter, no substantial densification of the material occurs in the subsequent stages beyond that to which the material has been subjected in the first stage.